1. Field
The present disclosure relates in general to the field of telecommunications and, more specifically to systems and methods for transmit diversity in wireless communication systems.
2. Description of the Related Art
Multi-antenna user equipment (UE) transmission is a key component of the current Long Term Evolution-Advanced (LTE-A) work in the Third Generation Partnership Project (3GPP). Because the current LTE frame structure time division duplex (TDD) may have many more downlink subframes than uplink subframes and because each of the downlink subframes carries a transport block, current LTE TDD supports transmission of up to four Ack/Nack bits in a subframe. These four Ack/Nack bits are transmitted using channel selection. More recently, it has been agreed that 3GPP use channel selection for up to four Ack/Nack bits to support carrier aggregation for both frequency division duplex (FDD) and TDD frame structures. Therefore, the use of channel selection for Ack/Nack feedback is of growing interest.
Ack/Nack bits are carried in LTE using physical uplink control channel (PUCCH) format 1a and 1b on orthogonal resources. Because no more than two bits can be carried in these PUCCH formats, two extra information bits for TDD are needed. These extra two bits are conveyed through channel selection. A UE encodes information using channel selection by selecting an orthogonal resource to transmit on. Channel selection uses four orthogonal resources to convey these two bits This can be described using Table 1 below:
TABLE 1PUCCH format 1b channel selectionCodewords 0 to 15Ack/Nack Information bits: b3b2b1b0RResDRes0000000100100011010001010110011110001001101010111100110111101111001j−j−10000000000001100001j−j−10000000022000000001j−j−10000330000000000001j−j−1
Each column of the table indicates a combination of Ack/Nack bits (or a “codeword”) to be transmitted. Each row of the table represents an orthogonal resource. Each cell contains a QPSK symbol transmitted on the orthogonal resource to indicate the codeword. The “DRes” column indicates which orthogonal resource carries the QPSK symbol, and the “RRes” column indicates the orthogonal resource used to carry the reference symbol. Note that each column of the table contains only one non-zero entry, since channel selection requires that only one resource is transmitted upon at a time on one transmission path.
For example, when Ack/Nack bits ‘0110’ are to be transmitted, the UE will transmit the QPSK data symbol ‘−j’ using orthogonal resource ‘1’. The reference signal transmission will also be on orthogonal resource ‘1’. LTE carries Ack/Nack signaling on format 1a and 1b of the physical uplink control channel (PUCCH) (as specified in section 5.4 of 3GPP TS 36.211 V8.8.0, “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8)”, September, 2009).
The subframe structure of PUCCH formats 1a and 1b with normal cyclic prefix is shown in FIG. 1. Each format 1a/1b PUCCH is in a subframe made up of two slots. The same modulation symbol d is used in both slots. Without channel selection, formats 1a and 1b set carries one and two Ack/Nack bits, respectively. These bits are encoded into the modulation symbol d, using BPSK or QPSK modulation, depending on if one or two Ack/Nack bits are used.
Each data modulation symbol, d, is spread with the sequence, ru,vα(n) such that it is by a 12 samples long (which is the number of subcarriers in an LTE resource block in most cases). Next, the spread samples are mapped to the 12 subcarriers the PUCCH is to occupy and then converted to the time domain with an IDFT. The spread signal is then multiplied with an orthogonal cover sequence wp(m), where mε{0, 1, 2, 3} corresponds to each one of 4 data bearing OFDM symbols in the slot. There are 3 reference symbols (R1, R2, and R3) in each slot that allow channel estimation for coherent demodulation of formats 1a/1b.
There are 12 orthogonal spreading sequences (corresponding to ru,vα(i) with αε{0, 1, . . . , 11} indicating the cyclic shift) and one of them is used to spread each data symbol. Furthermore, in Release 8 of LTE (Rel-8) there are 3 orthogonal cover sequences wp(m) with pε{0, 1, 2} and mε{0, 1, 2, 3}. Each spreading sequence is used with one of the orthogonal cover sequences to comprise an orthogonal resource. Therefore, up to 12*3=36 orthogonal resources are available. Each orthogonal resource can carry one Ack/Nack modulation symbol d, and so up to 36 UEs may transmit an Ack/Nack symbol on the same OFDM resource elements without mutually interfering. Similarly when distinct orthogonal resources are transmitted from multiple antennas by a UE, they will not interfere with each other, or with different orthogonal resources transmitted from other UEs. When there is no channel selection, the orthogonal resource used by the UE is known by the base station that serves it (called an enhanced Node B, or “eNB”). As discussed above, in case of channel selection, a part of the information bits determines the orthogonal resource to be utilized. The eNB detects that part of the information bits by recognizing what orthogonal resource is carrying other information bits.
Orthogonal resources used for reference symbols are generated in a similar manner as data symbols. They are also generated using a cyclic shift and an orthogonal cover sequence applied to multiple reference signal uplink modulation symbols. Because there are a different number of reference and data modulation symbols in a slot, the orthogonal cover sequences are different length for data and for reference signals. Nevertheless, there are an equal number of orthogonal resources available for data and for reference signals. Therefore, one can use a single index to refer to the two orthogonal resources used by a UE for both the data and reference signals, and this is done in Rel-8. This index is signaled in Rel-8 as a PUCCH resource index, and is indicated in the LTE specifications, including section 5.4 of 3GPP TS 36.211 V8.8.0, “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8)”, September, 2009, as the variable nPUCCH(1).
One cyclic shift may be used to transmit all symbols in a slot (including both data and reference symbols) associated with an antenna. In this case, the value of is constant over the slot. However, LTE Rel-8 also supports cyclic shift hopping, where α varies over the slot. Cyclic shift hopping transmissions are synchronized within a cell such that UEs following the cell-specific hopping pattern and do not mutually interfere. If neighbor cells also use cyclic shift hopping, then for each symbol in a slot, different UEs in the neighbor will tend to interfere with a UE in a serving cell. This provides an “interference averaging” behavior that can mitigate the case where one or a small number of neighbor cell UEs strongly interfere with a UE in the serving cell. Because the same number of non-mutually interfering orthogonal resources are available in a cell regardless of if cyclic shift hopping is used or not, it is possible to treat orthogonal resource equivalently for the hopping and non-hopping cases. Therefore, hereinafter when we refer to an orthogonal resource, it may be either hopped or not.
The LTE PUCCH format 1a/1b structure shown in FIG. 1 varies somewhat depending on a few special cases. One variant of the structure that is important to Tx diversity designs for format 1a/1b is that the last symbol of slot 1 may be dropped (not transmitted), in order to not interfere with sounding reference signal (SRS) transmissions from other UEs.
In the context of LTE-Advanced carrier aggregation, a UE may receive scheduling grants on the physical downlink control channels (PDCCHs) of multiple component carriers, identified as ‘primary’ or ‘secondary’ component carriers (these are also referred to as ‘primary cells’ and ‘secondary cells’, respectively). Hereinafter, we will use the abbreviation ‘PCC’ and ‘SCC’ to refer to the primary or secondary component carriers (or ‘cells’), respectively.
When a UE receives at least one scheduling grant for one transport block, but also does not receive a different grant for a second, simultaneously transmitted, transport block, it can indicate a Nack state for the second transport block. This use of one value for both Nack and when a grant is known to be missed is often referred to as signaling ‘Nack/DTX’ in 3GPP parlance, due to the similarity to the case where a UE would not transmit when it missed a grant for one transport block in Rel-8. Transmitting a Nack/DTX when a grant is missed allows a fixed number of information bits to be transmitted for Ack/Nack by the UE even when the number of transport blocks for which it receives scheduling grants varies. That is, when it receives two grants, it transmits two Ack/Nack bits, and when it misses a grant, it still transmits two Ack/Nack bits, but sets the one it missed the grant for to ‘Nack/DTX’.
Multi-antenna UE transmission is a key work item of the current LTE-Advanced (LTE-A) work in the Third Generation Partnership Project (3GPP). Uplink transmit diversity techniques are under consideration for standardization, and have been proposed for physical uplink shared channel (PUSCH), and all PUCCH formats.
Many of the uplink Tx diversity schemes would have one or more of the following drawbacks if they were to be applied to LTE Ack/Nack channel selection: 1) Use extra uplink resource, for example by transmitting the same symbol on different orthogonal resources; 2) increase the peak-to-average transmit power ratio (or “cubic metric”), leading to higher peak power requirements for the UE power amplifiers; 3) reduce robustness to multipath by requiring nearly the same channel response on different subcarriers; or 4) require an even number of OFDM symbols in a PUCCH slot.
There are two main classes of transmit diversity approaches that could be considered for use on 3GPP PUCCH format 1a/1b. The first class of methods described in (1) R1-091925, “Evaluation of transmit diversity for PUCCH in LTE-A”, Nortel, May 4-8, 2009, San Francisco, USA; and (2) R1-092065, “PUCCH Transmit Diversity”, Qualcomm Europe, May 4-8, 2009, San Francisco, USA is space-orthogonal transmit diversity, or “SORTD.” As can be seen in the two-antenna example in FIG. 2a, in this approach each antenna transmits a different orthogonal resource that carries the channel coded control information of PUCCH.
There are two variants of SORTD. In the first variant, the coded bits are duplicated before spreading with the orthogonal resource associated with each antenna. This is labeled as “simple repetition for SORTD” in FIG. 2b. This method provides maximum diversity gain in a flat fading channel, because the coded bits can be perfectly separated using the two orthogonal spreading sequences. The disadvantage of this approach is that two orthogonal resources are used, which means that half as many users can share the same PUCCH as compared with when each UE would only use one PUCCH orthogonal resource.
In the second variant called space-orthogonal spatial multiplexing or “SORSM” labeled as “Joint Coding for SORTD” in FIG. 2c, a lower rate encoder is used and different coded bits are transmitted on the orthogonal resources and antennas. This variant has better performance than the simple repetition approach due to the increased coding gain of the R/2 rate code. However, it shares the same disadvantage as the first variant: it requires twice the number of orthogonal resources used for 1 antenna transmission. This disadvantage is one of problems targeted by this disclosure.
The second class of methods is space time block codes, such as the “Alamouti” code, known to those of skill in the art. A forward error correction code is typically used, and then the coded symbols are modulated to form a symbol stream, ‘s’. These symbols are taken two at a time, and then the first symbol is transmitted on the first antenna at a first time, while at the same time the second symbol is transmitted on the second antenna. At a second time instant, the second symbol is negated and conjugated and transmitted on the first antenna, while at the same time the first symbol is conjugated and transmitted on the second antenna. Because the symbols are transmitted simultaneously on the two antennas, but also are transmitted over two time slots, the number of symbols transmitted on each antenna is the same as the number of symbols produced by the modulation and coding (that is, this is a “rate one STBC”). Due to the properties of the STBC, a receiver can recover the two transmitted symbols such that the power from the two antennas combines efficiently, and excellent diversity gain is achieved. This diversity gain reduces the chance of an erroneous reception, improving performance under difficult channel conditions.
Since STBCs often operate on pairs of symbols, their direct application to channel selection is difficult, because each PUCCH carries only one modulation symbol. A second problem is that the channel should not change between the two time instants. Because PUCCH formats 1a/1b are frequency hopped between slots, the channel varies significantly between the two slots. Since channel selection is used to allow transmission of more than 2 Ack/Nack bits, an alternative solution to support more than 2 bits is to modify format 1a/1b to carry multiple symbols in a slot by using a reduced spreading factor. However, we would prefer not to make such a significant change to the slot structure, especially since such a structure would likely have worse performance and there can be other problems, such as operation in cases where the last OFDM symbol in the second slot of PUCCH format 1a/1b may be dropped).